Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats

Latent Heat of Water.


The above formula gives P or V*at any temperature, when c
is known.

Three States of Matter.—These, the solid, liquid, and
gaseous, are well understood, and it is also now admitted that all
bodies are capable of existence in each state successively, though
not necessarily at the ordinary pressure and temperature. Taking
one pound of any substance and applying the specific heat due to
its state, its temperature rises one degree, and as the specific heat
is approximately regular for each state, practically the whole heat
is registered on the thermometer. But in all substances two
critical points occur called the points of fusion and evaporation,
and known respectively in the case of water as the (freezing and
boiling points;7 at these points additional heat is absorbed merely
to do the work of re-arranging the molecules, of fusing or melting
on the one hand, and of evaporating on the other hand. Such
'latent' heat is not observable on the thermometer, and must,
therefore, be otherwise detected.

Latent Heat is the quantity of heat units absorbed or given
out in changing one found of a substance from one state to another
without altering its temperature. This phenomenon, first observed
by Black about 1757, will now be demonstrated in the case of
water, and the units measured.

Latent Heat of Water is that required to melt one pound
of ice at 32° F. Provide a vessel with felt-covered sides, similar to
that at Fig. 598. Fill it with water of known weight (w) and tempera-
ture (t°). Take a piece of ice which has begun to melt, wipe dry,
weigh (0/j), place in the water, and close the apparatus. When
the ice is quite melted^ gently stir, and measure the final tempera-
ture (T°), which may be a few degrees above 32°. Let Lh = the
latent heat of water; then

Heat lost by water   =             Heat gained by ice

weight x fall of temp. « weight x (latent ht. + rise of temp.)
w(f-T)         -          ^{Lh+ CT-sO}-

Supposing 20 oz. of water at commencement, at 60*, and 2 oz,
of ice at, of course, 32°; the final temperature being 45°, then

20 (60 - 45) - 2 (Lh + 45 - 32)»

, r        300 - 26                .

and LI* ** a—__ « 137 units. •